[dmm]

What if the macronutrient content of your diet affects your metabolic rate? I mean I see the value of the "bucket with a hole in it" model of human weightloss but the body is a complex biological machine full of feedback mechanisms so maybe that model isn't always sufficient.

[kazinator]

If we are talking about control over body fat, it's not so much about metabolic rate, as about mobility of the stored fat. If your adipose fat does not respond to reasonable caloric restrictions, you're simply going to have a hard time.

It is not actually hard to burn mobilized fat. Each one of us burns a whole lot of dietary fat in a given year: a lot more than what we have stored in our bodies. Why is it that we can burn through all that dietary fat, far in excess of how much body fat we can burn in the same period? Because that fat it is mobile: it is circulating in the body in a form that is ready for use.

Thermodynamics provides us with a summary of what is going on, which doesn't reliably translate to a method. If we measure the total energy output of the body while and monitor the energy input, then the body mass and composition changes will be reliably related to those variables.

Not all body composition changes are favorable, though.

If you could simply cut 500 calories a day, and reliably have the deficit made up by burning body fat, it would be laughably easy for anyone to achieve a vanishingly low body fat level.

[jp555]

Macros do not significantly affect metabolic rate. Exercise does though.

Glycogen (carbs) is the preferred fuel for almost every cell in our bodies. All the fat & protein we eat that ends up used as metabolic fuel will first be converted into carbs. Now granted the thermal effect of these macros are different (it takes more energy to turn protein into fuel than it does to convert table sugar) but the idea that "you need to eat fat to burn fat" is a myth.

[cschmidt]

Many people think that ketones are the preferred fuel for every cell in our bodies.

[tomp]

Do they? Everything I've read about keto diet is that it will make you less "short-term energized" - you're lacking glycogen, i.e. you won't be able to sprint as much.

[jp555]

everyone is entitled to their own beliefs, but not their own facts. Glycogen is the preferred fuel, which are giant chains of glucose (carb) molecules. I've done keto, and you have to be very careful as even a small amount of carbs will quickly shift the body out of ketosis and back into using glycogen. Doesn't the fact that ketosis is so fragile indicate to you that it's not the preferred state? Homeostasis reigns, and the body gets away from ketosis whenever it can. It's an extreme state.